In computer networks, the use of redundant communication links has become a primary means of insuring that system down time is minimized when a communication link is lost. in some networks, the failure of the physical medium which comprises the communication link is a primary source of network problems (e.g., down time). Such failures, coupled with the increased use of local area network architectures, have increased the demand for allowing for a timely selection of one of a number of redundant links.
Recognizing the need for redundant links within communication networks, system designers have attempted to devise switching circuits, external to the communication units that exchange data over these links, which allow the individual communication units to be selected. Unfortunately, this external logic requires more components and is more expensive.
FIG. 1a illustrates a conventional scheme for selecting among redundant communication units/links. In this system, communication unit 105 and communication unit 106 may each be associated with a single network unit (e.g., a server or other device which may be part of a computer or other communication network) but are not directly coupled to each other. Instead, the communication units are coupled to one another through an external switching circuit consisting of voltage source 104, two switches 102 and 103 and power supply 101. In this way, each communication unit signals its availability to the external switching circuit without notifying the other communication units. For example, if communication unit 105 is active, it transmits a signal to voltage source 104, which, in turn, activates switch 102. Switch 102 will close and complete the circuit between power supply 101 and communication unit 105. As a result, communication unit 105 will be enabled so that it may receive or transmit signals on data line 107. Alternatively, if communication unit 106 is active, switch 103 will be closed (e.g., in response to a signal from voltage source 104), allowing communication unit 106 to be enabled for communication over data line 108.
FIG. 1b illustrates another conventional configuration for a communication system employing redundant communication links. In this configuration, an external arbiter 110 is used to observe signals produced by one or more communication units which indicate that the respective communication unit has detected an active link. In such a configuration, upon receipt of an indication that a particular communication unit has detected an active communication link, the arbiter 110 may enable an appropriate pair of pass gate transistors (e.g., transistor pair Q1 and Q2 or transistor pair Q3 and Q4) to allow PHY 112 to communicate via the active link. In yet another alternative configuration, such as is shown in FIG. 1c, a PHY 114 may be configured to perform its own internal selection of a communication unit upon receipt of a signal indicating that a communication unit has detected an active link.
Configurations such as those shown in FIGS. 1a and 1b require additional, external switching circuitry for each new communication unit/data line pair. It would be desirable to have a system, expandable to any number of links, that avoids such limitations. Further, although systems such as that shown in FIG. 1c avoid the need for external circuitry, such systems require complex PHY units (e.g., PHY 114) which include on-board arbitration circuitry or the like. It would be desirable to avoid the need for such complex PHY units in a system employing redundant communication links.